In the oil and gas industry, tubular members installed axially into the earth are used for a variety of purposes. For example, tubular members are frequently used as foundation piles to support the weight of an offshore structure and to resist environmental loads applied to the structure. Tubular members are also used as well conductors to facilitate the drilling of wells from an offshore platform. Other uses of tubular members will be well known to those skilled in the art.
Typically, the objective is to install the open-ended tubular member into the earth a distance, known as the target penetration, which is sufficient to mobilize the required load carrying capacity of the tubular member. Failure to mobilize the desired load carrying capacity of a tubular member means that the installed tubular member may not be fit for its intended purpose because it may not be able to resist the applied loads. If the tubular member is a well conductor, another objective is to preclude soil fracture during subsequent drilling operations. The ability of the subsurface soils to withstand fracture is known as "fracture integrity". Fracture integrity may be either local or global. Local fracture integrity refers to the ability of the soils to withstand fractures along the interface between the conductor and the surrounding soils. Global fracture integrity refers to the ability of the soils to withstand fractures at some distance from the wall of the conductor or below the top of the conductor. Compromising the fracture integrity of the surrounding soils can lead to lost returns and, potentially, to loss of the well during subsequent drilling operations.
Generally, there are two fundamental ways in which a tubular member may be installed into the earth. First, the tubular member may be installed into the earth in the manner of conventional foundation piles. Second, a borehole may be drilled into the earth and the tubular member cemented therein. The drilling of wells in the deep waters of the Gulf of Mexico ("GOM") is often problematic because of the presence of over-pressured (excess water pressure) sands or soils which are often found at relatively shallow depths [e.g., 1000 to 2000 feet below the mudline ("BML")]. These sands and/or soils may be deposited in one or more layers or regions (not by way of limitation, these regions are hereinafter referred to as "sand regions" and can include sand, soil or a mixture thereof) and are typically surrounded by clays that are normally to under-consolidated. A conventional tubular well casing with an open flow area typically cannot penetrate deep enough to drive through the over-pressured sand region (or through multiple over-pressured sand regions) as well as through a sufficient interval of the overlying and underlying clays to maintain adequate pressure integrity both above and below the over-pressured sand region(s). The penetration of a conventional driven casing is generally limited because of a combination of low driving impedance and insufficient hammer energy. Driving impedance is a measure of the transmissibility of a stress wave through a medium, in this example the well casing. It is defined by the equation: I.sub.d =EA/c, where I.sub.d =driving impedance, E=modulas of elasticity of the medium, A=cross-sectional area; and c=wave speed. As described further below, a conventionally driven standard tubular casing with an open flow area will not likely be satisfactory for driving through over-pressured sand regions.
Drilling through over-pressured sand regions with conventional methods will often result in sand and water flowing to the mudline both through the flow area of the well casing and the interface between the well casing and the formation. These incidents of sand flow can result in loss of casing support and the wells due to buckling and possible subsidence of the seafloor. Should significant sand flow occur after siting the production facility, the resulting subsidence may adversely affect the capacity of the foundation and may ultimately lead to catastrophic foundation failure or abandoning the facility. The consequences of these outcomes can result in significantly higher drilling and production costs and lost income. It is conceivable that under extreme conditions it may not be economical to produce a field because of this problem.
The current technology for drilling through over-pressured sand regions uses a combination of three-dimensional ("3-D") high-resolution shallow seismic data and standard well drilling techniques that are adapted to the geological conditions. The state of the practice is to identify with the 3-D data the aerial extent of the over-pressured formation and to either avoid the formation or select a location where the over-pressured region is thinnest. It is not possible however to determine the relative pressure in the formation with seismic data. Accordingly, once the drilling location is chosen, drilling proceeds conventionally with emphasis on drilling through the over-pressured sand layers as quickly as possible using drilling mud or seawater with gel sweeps.
When attempting to drill with mud at equilibrium conditions (no flow in or out of the borehole), efforts are made to control any over-balance zones (where sand flows into the borehole) in the formation by injecting heavy mud (dynamic kill). Other drilling technology that has been tried includes injecting a monomer or polymer cement into the over-pressured sands to stem the water flow prior to drilling the casing hole. After drilling is complete, the casing string is cemented to the formation and to the prior casing strings in the conventional manner to prevent flows around the outside of the casing and within the annulus between casing strings.
Practically speaking, it may not be possible to obtain equilibrium of fluid pressures in the borehole even using a riser, because the equilibrium pressure may be very close (within 0.5 lb./gal.) to the fracture pressure. Exacerbating this condition may be several thousand feet of drill cuttings in the fluid column that cannot be precisely controlled and add to the mud weight. Thus, if the borehole fluid pressure is less than the formation pressure, sand flows into the borehole and wellbore stability is a problem. If the borehole fluid pressure is greater than the formation pressure, then he fluid pressures in the borehole will fracture or liquefy the sand formation. Ultimately, this could lead to communication between wells or to fluid pressures well n excess of hydrostatic reaching higher elevations and subsequently fracturing the formation to the seafloor. If this fracture intersects the foundations, premature failure of the foundation could occur.
Controlling the wellbore fluids to achieve pressure equilibrium is not sufficient to assure well and foundation integrity. An adequate cement bond is necessary between the casing and the borehole walls to prevent broaching of the over-pressured sands along the outside of the casing and to the seafloor. Standard well drilling practice is to drill a borehole larger than the casing and then to fill the void between the casing and the soil with cement. Current cementing technology does not assure that this void can be adequately filled, unless the hole is near gage. This is particularly critical for wellbores that have experienced flow or wall instability since an irregular wall profile exacerbates cementing difficulties and the lack of an adequate cement job may result in buckling of the well casing. When drilling underbalanced (i.e., not sufficient drilling fluid weight to prevent inflow of formation fluids) in granular soils the formation fluid will flow into the wellbore transporting the formation soils. If a sufficient volume of soil is removed over a sufficient interval of casing and a compressive force is then applied to the casing, the casing will buckle. This series of events occurred during the drilling of production wells at a deepwater GOM location, resulting in the site being abandoned.
For the foregoing reasons, installation of a tubular member through one or more over-pressured sand regions of the earth can be difficult to achieve and time consuming using current methods. There can also be significant adverse consequences resulting from installing tubular members through these over-pressured regions, such as compromising the fracture integrity of the formation or subsidence of the sea-floor. The present invention is aimed at providing a practical, economical and time efficient method for installing tubular members through over-pressured sand region(s) of the earth which will prevent sand or soil flow during installation and assure long term integrity of the well and production facility foundation.